Studies on DNA replication of plasmid ColE1 and its relatives have been continued. Initiation of plasmid DNA replication by RNA polymerase, ribonuclease H and DNA polymerase I have been studied. The mechanism of inhibition of primer formation by plasmid specific small RNA was elucidated. The nucleotide sequences of the regions that include the origin of DNA replication of plasmid RSF1030 and p15A were determined and the comparative studies on nucleotide sequences of ColE1 related plasmids were carried out.